In recent years, development of close proximity wireless transfer technology is accelerated. Close proximity wireless transfer technology enables communication between two devices in close proximity. Each device having a close proximity wireless transfer function includes a coupler. If two devices are brought closer within a transfer range, couplers of the two devices are electromagnetically coupled. These devices can wirelessly transmit and receive a signal to and from each other.
A typical coupler includes, for example, a coupling element, an electrode pole, a resonant stub, a ground plane, and the like. A signal is supplied to the coupling element via the resonance stub and electrode pole. As a result, an electric current flows in the coupling element and an electromagnetic field is generated around the coupler. This electromagnetic field enables an electromagnetic coupling between the couplers of the two devices brought closer to each other. Another example of the typical coupler is an inverted-F antenna.
Incidentally, in the coupler, a sufficient tolerance for position shifts between the coupler and a partner coupler is required. This is because wireless transfer between devices should not be affected even if the positional relationship between the devices in close proximity is slightly shifted.
Further, a coupler contained in a device is required to have high impedance. This is because if the coupler is mounted in the device, coupling arises between the coupler and peripheral components in the device, reducing the input impedance of the coupler. The reduced input impedance could lead to a degraded electromagnetic radiation efficiency of the coupler.
Still further, in recent years, a lower height of a coupler is requested so that the coupler can be mounted in various devices.